Inter‐Decadal Variations and Causes of Summer Heavy Precipitation in the Water Tower Area of China

ABSTRACT Heavy precipitation events have a greater impact on ecology, economy, and human livelihoods than general precipitation events. Based on observational data from 19 meteorological stations, two gridded daily precipitation datasets, and the ERA5 monthly reanalysis dataset from June to August during 1961–2022, this study investigates the inter‐decadal variations of summer heavy precipitation and its underlying mechanisms in the Three‐River Source (TRS) region, known as the water tower of China. The results show that over the past 60 years, summer heavy precipitation in the TRS has exhibited an overall increasing trend, with a significantly stronger trend in the eastern TRS (98°–102° E) than in its central and western parts. The interannual variation rate is relatively larger in the first three decades, but summer heavy precipitation in the latter 30 years initially decreases before increasing again. Since the 21st century, a Silk Road‐like wave train characterized by positive–negative–positive geopotential height anomalies (anticyclone–cyclone–anticyclone) has dominated the middle and upper troposphere over Eastern Europe, Central Asia, and East Asia. During this period, the western Pacific subtropical high has intensified and extended westward, while the South Asian high has strengthened and extended eastward, creating a synoptic background that favours the interdecadal increase in summer heavy precipitation in the TRS. Since the 21st century, the westerly jet has intensified, shifted northward, and extended eastward, contributing to enhanced upper‐level divergence and lower‐level convergence over the TRS region located south of the jet core. Additionally, a notable rise in spring sensible heat (SH) around 2003 strengthened the thermal pumping effect over the TRS, which, combined with increased atmospheric instability, provided favourable thermodynamic lifting conditions for heavy precipitation events in the region since the 21st century. In addition, the inter‐decadal variation of the regional net water vapour budget follows a similar pattern, showing a transition from less to more around 2002. The strengthened anticyclonic circulation in Northeast Asia has reduced the outflow of water vapour from the eastern boundary of the TRS. Meanwhile, intensified southwesterly water vapour transport and enhanced water vapour convergence have further contributed to the interdecadal increase in both summer heavy precipitation and the net water vapour budget in the TRS since the 21st century.